Electric valve protective system



Sept. 16; 1947. s, DURAND 2,427,450

ELECTRIC VALVE PROTECTIVE SYSTEM Filed- 001;, 24,' 1942 2 Sheets-Sheet 1 Q-' MW 2 24m Sept. 16, 1947. v s, @DURAND 2,427,450

ELECTRIC VALVE PROTECTIVE SYSTEM Filed Oct. 24, 1942 2 Sheets-Sheet 2 Pa tented Sept. 16, 1947 UNITED STATES PATENT OFFICE ELECTRIC VALVE PROTECTIVE SYSTEM Samuel R. Durand, Milwaulree, Wis., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a corporation of Delaware Application October 24, 1942, Serial No. 463,160

Claims. 1

This invention relates in general to improvements in electric valvecontrol systems, and more particularly to means for controlling the excitation of a group of electric valves having separate cathodes in such manner as to render the valves conductive or nonconductive in accordance with the requirements of a converting system of which the valves are a part.

Electric current converting systems frequently comprise a group of valves each provided with a single main anode and with a separate vaporizable cathode. To maintain such a system in operation it is generally preferable to maintain the different cathodes continuously emissive even when they are required to operate only intermittently. Upon occurrence of a disturbance such as a backfire in one of the valves, the flow of current through the different valves may be interrupted by means of control electrodes of the so-called grid type. The action of the control electrodes, however, is not always absolutely reliable and it is therefore preferable to rely on the interruption of the excitation currents of the cathodes to render the cathodes nonemissive and to utilize the action of the control electrodes at the same time for deionizing the valves and thereby prevent any abnormal reignition thereof.

The excitation currents of all the cathodes should be interrupted simultaneously and as rapidly as possible. In general, continuous excitation current is obtained from a source of alternating current through a transformer and through connections which may include rectifying devices, at least when a single excitation anode is associated with each cathode. The connections between the excitation transformer secondary winding and the valve electrodes are preferably made inductive to render the excitation current more uniform and to render the excitation are more stable. If excitation is interrupted by switching means controlling the current between the source and the transformer primary winding, opening of the switching means does not result in immediate interruption of the excitation current. The reason therefor is that the transformer secondary circuit remains closed, so that the excitation current is able to continue to flow therethrough for an appreciable length of time to discharge the magnetic energy stored in the inductive means forming part of the secondary circuit. It is therefore preferable to provide for opening the secondary connections to insure interruption of the excitation current immediately upon opening of the switching means.

In converters provided with automatic control it is generally desirable simultaneously to interrupt the excitation currents and to cause opening of a switch controlling the connection of the valves with the supply circuit or with the load circuit. The excitation currents may then be reestablished after a predetermined interval following opening of the switch, and re-establishment of the excitation currents may also be utilized for causing reclosure of the switch.

It is therefore one of the objects of the present invention to provide a control system for a plurality of electric valves of the continuously excited cathode type, by means of which the oathodes may be rendered simultaneously nonemis- 'sive in the shortest possible time.

Another object of the present invention is to provide a control system for an electric valve of the continuously excited cathode type utilizing the cooperative action of control electrode energization and of excitation current interruption to effect interruption of the flow of current through the valve.

Another object of the present invention is to provide a control system for a plurality of electric valves of the continuously excited cathode type by means of whichthe valve excitation currents may all be interrupted within a predetermined time interval regardless of the inductance of the excitation circuits.

Another object of the present invention is to provide a control system for an electric valve of the continuously excited cathode type for coordinately controlling the excitation of the valve and the operation of a. switch controlling the connection of the valve with a supply circuit or with a load circuit.

Objects and advantages other than those above set forth will be apparent from the following de scription when read in connection with the accompanying drawing, in which:

Fig. l diagrammatically illustrates one embodiment" of the present invention applied to the control of a group of six valves forming part of an alternating current rectifying or direct current inverting system; and

Fig. 2 diagrammatically illustrates a modification of a part of the excitation circuits of the embodiment illustrated in Fig. 1.

Referring more particularly to the drawing by characters of reference, reference numeral 6 designates a polyphase alternating current circuit to be connected with a direct current'circuit I through a converting system comprising a transformer 8 and a, plurality of interconnected electric valves 9 to 14 inclusive. Either circuit 6 or circuit 1 may be chosen as supply circuit for the system, but it will be assumed that circuit 6 is a supply circuit energized from a suitable generator (not shown). Circuit 1 is accordingly a load circuit transmitting current to suitable load devices (not shown).

Transformer 8 comprises a primary winding 16 subdivided into a plurality of phase portions connectable with circuit 6 through a switch I1 serving to control the connection between circuits 6 and 1 through the valves. Winding I6 is inductively related with a secondary winding 18 constituting a network divided into a plurality of pairs of opposite phase portions. The different pairs of phase portions of winding l8 are severally connected with one of the conductors of circuit 1 through pairs of valves such as valves 9, I2. Winding I8 is preferably arranged to define a plurality of neutral points severally connected with the other conductor of circuit 1 through an interphase transformer l9.

Valves 9 to I4 are preferably of a uniform design, each valve comprising a casing which may be made of insulating or conductive material. Valve 9 comprises a cathode 20 consisting of a pool of a suitable liquid conductor such as mercury which may be insulated from the associated casing 2| when the casing is made of conductive material. Valve 9 further comprises a single main anode 22 cooperating with cathode 20 for the flow of current between circuits 6 and 1 connected therewith.

Valve 9 may be rendered conductive by an ignition system comprising an ignition anode 23 supported a short distance above the surface of cathode 20. Current may be supplied to anode 23 from circuit 6 through connections comprising the usual step down transformer 26, a switch 21, an auxiliary circuit 25 and the primary winding 28 of an ignition-excitation transformer 30. The secondary winding 29 of transformer 30 is divided into a plurality of independent phase portions each provided with two terminals and a midtap or neutral point. The terminals of winding 29 are connected with a six-phase rectifying device 3|, and ignition current is supplied to anode 23 from the different phase portions of winding 29 operating in sequence through rectifier 3|, contacts 32a of an excitation relay 32, and a resistor 33. The ignition current returns to Winding 29 through cathode 20, the common connection between the cathodes, contacts 350, c and 35g of a relay 35, and a plurality of reactors 34 severally connected with the phase midtaps of winding 29.

Relay 35 is preferably chosen of a fast operating type provided with such arc extinguishing means that the current fiow through the contacts is completely interrupted upon full opening of the relay contacts, even if the circuits controlled by the relay are highly inductive. The relay may be of a type in which the closed contacts open before the open contacts close or may be of the so-called make before break type in which the open contacts close before the closed contacts open. Regardless of the type of relay utilized, the operation of all the contacts should take place within a time interval which is very short relative to the operating period of each valve so that all contacts may be considered to operate simultaneously.

Momentary engagement of anode 23 with cathode 20 to establish a cathode spot renderin the cathode emissive is controlled by means of a solenoid 36 which maybe energized from wind- 4 ing 29. Solenoid 36 is preferably connected with only one phase portion of winding 29 through contacts 31a of a control relay 31 and contacts 321) of relay 32. Solenoid 36 may cause engagement of anode 23 with cathode 20 in any known manner. For example, this result may be obtained by the solenoid attracting an armature forming a piston projecting a jet of cathode material against the anode.

Valve 9 is provided with excitation anodes 38 connected with the terminals of one of the phase portions of winding 29 through the left hand portions of a pair of paralleling reactors 39, the coils of relay 32 and a pair of resistors 40. Further connections extend from the right hand portions of reactors 39 through the coils of an excitation relay 4| similar to relay 32 to the excitation anodes of valve I2. Anode 22 and excitation anodes 38 are preferably connected with phase portions of windings I8 and 29 respectively of such phase relation that the flow of current is transferred from one excitation anode 38 to the other once during each conductive period of anode 22 and once toward the end of each idling period of anode 22. In this manner both current transfers are removed from the period of transient conditions immediately following each conductive period of anode 22 and the stability of the excitation arc is thereby improved.

The conductivity of valve 9 for the flow of current between anode 22 and cathode/20 may be controlled by means of a control electrode 42. Valve 9 may be rendered and maintained nonconductive by impression on electrode 42 of a deionizing negative potential from a suitable source of direct current, such as a generator 43, through a voltage divider 44. Generator 43 may be driven by any suitable means such as a motor 50 energized from circuit 25. Control electrode 42 is preferably connected with the movable tap of voltage divider 44 through a resistor 45 and through one of the phase portions of the secondary winding 46 of a transformer 41. The primary winding 48 of transformer 41 may be energized from circuit 25 through current limiting resistors 49 to cause winding 46 to impress on control electrode 42 a variable potential component. The composite potential impressed on control electrode 42 from voltage divider 44 and from winding 46 serves to control the conductivity of valve 9 during normal operation. A tertiary winding 5| of transformer 41 may be short 'circuited by contacts 35] and 35h to render transformer 41 inoperative.

Valves l0 to 14 inclusive are provided with electrodes similar to those of valve 9 and connected as well as controlled in a similar manner. The cathodes of the different valves are connected with the positive conductor of circuit 1 through a switch 52. The solenoids controlling the ign'ition of the different valves may be simultaneously connected with winding 29 and may likewise be simultaneously disconnected from winding 29 by means of contacts 31a, 31b, 310 of relay 31 for causing simultaneous initiation of the flow of current through the different valves.

To supply the necessary current impulses to the solenoids, relay 31 is alternately energized and de-energized during predetermined time inter- 'vals by suitable timing means. The timing means may comprise a timing relay 55 for connecting the coil of relay 31 with circuit 25. Relay 55 is illustrated conventionally On the drawing as it may be of any suitable known type. In order, however, to permit a convenient adjustment of nected withcircuit 25 through contacts 350, of

relay 35, contacts 32d of excitation relay 32 and similar. contacts of the other excitation relays, and through the contacts of a second timing relay 56 similar to relay 55. The coil of relay 56 is preferably energized from a rectifying device 51 and a transformer 58 having its primary winding connected with circuit 25 through contacts 35a, contacts 32d and other similar excitation relay contacts, and contacts 55a of relay 55. The coils of relays 55, 56 are preferably designed for operation at relatively low voltage to reduce the cost of rectifying devices 53, 51. Contacts 55a, 56a are then preferably inserted in the primary circuits of transformers 54, 58, which generally operate at a higher voltage, to minimize the effect of the contact resistance. Contacts 35a with contacts 32d and the similar contacts also control the connection between circuit 25 and the coil of a lockout time delay relay 59. The contacts of relay 59 control a connection between transformer 26 and the coil of switch 21 through a hand operated switch 66.

The operating coil 35m of relay 35 is energized in response to the magnitude of the flow of current through transformer 8 by means of one of a bank of current transformers 6|. The current transformers are connected with overload relays 62 which operate in a slightly longer time than relay 35 and are therefore conventionally shown as time delay relays. Relays 62 control the energization of a holding coil Nb of switch" serving to maintain the latch We of the switch in engaged position.

The trip coil 520 of switch 52 may be connected with circuit 25 through contacts 35d or through the contacts of a reverse current relay 63 responsive to the direction of flow of current in circuit 1. Switch 52 may be provided with auxiliary contacts 52d for controlling a time delay reclosing relay 64.

The system being connected as shown on the drawing and circuit 6 being energized, operation may be initiated by closing switch 60. The coil of switch .21 thereupon receives current from transformer 26 through the contacts of relay 59 and through switch 60, thereby causing switch 21 to connect circuit 25 with transformer 25. Current is then supplied from circuit 25 to transformer 54 through contacts 35a, 3261 and 56a. Transformer 54 and rectifier 53 supply current to the coil of relay 55 which closes its contacts. Contacts 55a. connect transformer 58 with cir cuit 25 through contacts 35a and 32d. Transformer 58 and rectifier 51 energize the coil of relay 56, which opens contacts 56a. The coil of relay 55 is thereby de-energized and, after a predetermined time delay, relay 55 returns to the position shown. Contacts 55a are thereby opened to de-energlze the coil of relay 56. After another predetermined time delay, relay 56 also returns to the position shown. The above described se-- quence of operation of the two relays is continually repeated as long as the relay coils remain supplied with current.

Each operation of relay 55 causes alternate closure and opening of contacts 55b, thereby causing intermittent operation of relay 31. When relay 31 closes its contacts, contacts 31a connect one of the phase portions of winding 29 with solenoid 36 through contacts 32b, and contacts 3112,

. 34 back to winding 29. When relay 31 opens its contacts the solenoids are simultaneously disconnected from winding 29 to cause simultaneous interruption of the jets of cathode material. Arcs are thereby simultaneously drawn between the ignition anodes and the cathodes, thereby establishing cathode spots rendering the cathodes emissive. If for any reason, however, a cathode spot is not established at one of the cathodes, the associated solenoid continues to receive current impulses through the corresponding set of contacts of relay 31 until the cathode is rendered emissive.

When cathode 20, for example, has been rendered emissive, alternate half waves of current flow from winding 29 through the two coils of relay 32, resistors 40, excitation anodes 39, cathode 20, contacts 350 and one of reactors 34 back to winding 29. The excitation current impulses are caused to overlap by the inductance of reactor 34 so as to render the current through cathode 20 continuous and sufficiently uniform and stable to maintain the cathode continuously emissive. The flow of current through the coils of relay 32 causes the relay to operate to open the circuits of solenoid 36 and of ignition anode 23. During the ignition operation of the valves, current is also supplied from circuit 25 to the coil of relay 59 through contacts 35a and through excitation relay contacts such as 32d.

If all of the excitation currents have become established before relay 59 has operated, the excitation relays such as 32 all operate to de-energize relays 59, 55 and 56. Switch I1 may then be closed and latch [10 is held engaged by holding coil Hi) to maintain switch 11 closed. Closure of switch 11 also results in'energization' of the closing coil 52a of switch 52 through contacts Ila and through serially connected contacts such as 320 of the excitation relays. Switch 52 is closed by coil 52a and is held closed by latch 521). Voltage divider 44 is so adjusted that winding 46 is able to render valves 9 to II periodically conductive in sequence, andupon closure of switches i1 and 52 the flow of current is established between circuit 6 and circuit 1 through the valves as is well known.

During normal operation of the system, current impulses are established in sequence through the different valves and combine in conductors 1 to form a now of substantially uniform direct current supplied to the load. If at any time the flow of excitation current fails in one of the valves, valve 9 for example, relay 32 returns to the position shown. Relays 55, 56 and 59 are thereby re-energized as above described and the ignition operation is repeated in valve 9 only. If ignition cannot be established initially within a predetermined time interval following closure of switch 21, or if it cannot be reestablished within an equal time interval following de-energization of relay 32, relay 59 opens its contacts to cause opening of switch 21. Circuit 25 is thereby de-energized and the flow of current through valves 9 to M is interrupted because of lack of excitation current. Coil l'lb is also de-energized and latch He is released, thereby causing switch H to be opened by spring l'ld.

Normal operation may also be affected by a disturbance such as a backfire in one of the valves. During a backfire, one of the valves becomes conductive for a reverse flow of current, and the sound valves supply current to the faulty valve to thereby short circuit winding l8. Circuit 6 then transmits to transformer 8 current of excessive magnitude and the current supplied to coil 35m from current transformer reaches such value as to cause operation of relay 35. The relay opens contacts 350, 356, 359. Each set of contacts interrupts simultaneously the excitation currents of a pair of valves, such as 9, l2, and the excitation current-s of all the valves are interrupted substantially simultaneously. If this interruption were effected by means of switch 21, reactors would maintain the excitation currents through the valves after opening of switch 21 for a length of time sufficient to dissipate the magnetic energy stored within the reactors. Contacts 35c, 35c, 35g, however, are serially connected with reactors 34, and the fiow of current through the reactors and through the excitation anodes is interrupted immediately upon opening of the contacts regardless of the inductance of the reactors. All the cathodes, except those momentarily carrying load or fault current, are thereby rendered nonemissive substantially simultaneously. The cathodes carrying load or fault current also become nonemissive at the end of their operating periods.

Substantially at the same time as it interrupts the excitation currents, relay 35 short circuits winding 5| through contacts 35), 35h. The voltages appearing in winding 46 are thereby caused to collapse. The deionizing negative potential thereupon remaining impressed alone on the control electrodes from voltage divider 44 is thereby, in effect, substituted for the variable composite potential previously impressed on the control electrodes from voltage divider 44 and winding 45. When the cathodes have become nonemissive, the normal current impulses through the different valves are no longer established. But the flow of excessive current having taken place through the valves from the inception of the backfire until interruption of the excitation currents causes the conductive vapor contained in the valve casings to be in a highly ionized condition. The result of this abnormal ionization is that, in spite of the lack of excitation current, some of the valves may reignite or even backfire. Impression of a deionizing negative potential on the control electrodes causes rapid deionization of the vapor and thereby substantially precludes accidental reignitions or additional backfires in the valves. The flow of current from transformer 8 through the sound valves to circuit 1 and to the backfiring valve is thus interrupted in the shortest possible time interval. 1

Upon interruption of the excitation currents, the excitation relays return to the position shown, thereby de-energizing closing coil 5211. Operation of relay 35 causes energization of trip coil 520 through contacts 35d, and latch 52b is thereby withdrawn to cause opening of switch 52. The trip coll may also be energized through the contacts'of relay 63 in response to a flow of reverse current to the backflring valve from a load having a back E. M. F. or from other generators or converters connected with circuit 1. Switch 52 closes auxiliary contacts 52d, thereby energizing the coil of relay 64. After a predetermined time delay following opening of switch 52, relay 84 connects coil 351:: with circuit 25 through contacts 35b, which are then closed. Coil 35k resets relay 35 to the position shown on the drawing. The ignition operation of the valves is thereby again initiated in the manner above set forth. When excitation is re-established in all the valves, the circuit of closing coil 52a is again completed by the excitation relays and switch 52 recloses to restore the system to its normal operating condition.

If, however, the disturbance persists as a result of failure of any element of the system to perform its function, the continued flow of excessive current in circuit 5 and through current transformers 8i causes operation of relays 62. The relays open the circuit of holding coil llb and switch I1 is opened by spring lld.

To simplify the design of valves 9 to H, it is desirable to omit the excitation anodes and to supply direct current continuously to the ignition anode, which then also functions as excitation anode. This simplification, however, generally requires an increase in excitation current. The above described system may then again be used with only the modifications shown in Fig. 2. Each excitation relay such as 32 is then provided with only one coil, and the contacts such as 32a are omitted. The phase portions of winding 29 and rectifier 3| may remain as shown in Fig. l or may be reconnected as shown in Fig. 2 for the more advantageous full wave operation to supply only direct current to the ignition-excitation anodes.

The individual connections between the ignition-excitation anodes and winding 29 are preferably made inductive by means of reactors such as 65 to render the excitation currents more uniform and stable. The different connections may be controlled by a single set of contacts of relay 35, but are preferably joined in pairs to contacts 35d, 35f, 35h to permit the use of contacts of relatively small current carrying capacity as in the embodiment illustrated in Fig. 1. Except that ignition current is maintained in each valve to serve as excitation current, the operation of the present embodiment is identical to that of the embodiment illustrated in Fig. 1. By simultaneously opening the connections between rectifier 3| and the ignition-excitation anodes, relay I5 simultaneously interrupts the excitation currents of the different valves regardless of the inductance of the reactors such as 85.

The cathodes of the idling valves are thereby simultaneously rendered nonemissive and the others become nonemissive at the end of their operating periods as in the embodiment illustrated in Fig. 1.

Although but twoembodiments of the present invention have been illustrated and described, it will be apparent to those skilled in the art that various changes or modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secureby Letters Patent:

1. The combination of a supply circuit, a load tion between said circuits through said valve,

means for rendering said cathode emissive comprising a source of current connected with said cathode and with said excitation anode means to supply said cathode with excitation current,'

means for interrupting said excitation current and for causing opening of said switch, and time delay means for causing re-establishment of said excitation current in response to opening of said switch.

2. The combination of a supply circuit, a load circuit, an electric valve comprising a single main anode and a cathode connected with said circuits for the flow of energy therebetween and excitation anode means, means for rendering said cathode emissive comprising a source of current connected with said cathode and with said excitation anode means to supply said cathode with excitation current, switching means responsive to an operating condition of said valve for interrupting said excitation current, and means for causing said switching means to re-establish said excitation current with a predetermined time delay in response to current interrupting operation of said switching means.

3. The combination of a supply circuit, a load circuit, an electric valve comprising a single main anode and a cathode connected with said circuits for the flow of energy therebetween and excitation anode means, a switch for controlling the connection between said circuits through said valve, means for rendering said cathode emissive comprising a source of current connected with said cathode and with said excitation anode means to supply said cathode with excitation current, switching means for interrupting said excitation current and for causing opening of said switch, time delay means for causing said switching means to re-establish said excitation current in response to opening of said switch, and means responsive to re-establishment of said excitation current for causing reclosure of said switch.

4. In a control system for a plurality of electric valves each comprising a single main anode and a cathode for the now of nonconsecutive load current impulses, means for rendering said cathodes emissive comprising excitation anodes associated with said cathodes and a source of alternating current, the combination of a transformer comprising a primary winding connected with said source and a secondary winding having terminals and a neutral point, connections between said terminals and said excitation anodes, a further connection between said cathodes and said neutral point, and means for simultaneously rendering said cathodes nonemissive to thereby interrupt the flow of said load current impulses comprising means responsive to an operating condition of said valves for opening said further connection.

5. The combination of a plurality of electric valves each comprising an anode and a cathode, means for rendering said cathodes emissive comprising excitation anodes associated with said cathodes, a source of alternating current, a transformer comprising a primary winding connected with said source and a secondary winding having terminals and a neutral point, connections between said terminals and said excitation anodes, a further connection between said cathodes and said neutral point, inductive means in said further connection for rendering the flow of current therethrough more uniform, and means for simultaneously rendering said cathodes nonemissive comprising switch means for opening said further connection, whereby the flow of current through said excitation anodes is interrupted immediately upon interruption of the flow of cur- ,rent through said switch means regardless of the inductance of said inductive means.

6. In a control system for a plurality of electric valves each comprising a single main anode and a cathode for the flow of nonconsecutive load current impulses, means for rendering said cathodes emissive comprising excitation anodes associated with said cathodes and a source of alternating current, the combination of a transformer comprising a primary winding connected with said source and a secondary winding, connections between said secondary winding and said excitation anodes, a further connection between said secondary winding and said cathodes, and means for simultaneously rendering said cathodes nonemissive to thereby interrupt the flow of said load current impulses comprising means responsive to an operating condition of said valves for opening said further connection.

'7. In a control system for a plurality of electric valves each comprising a single main anode and a cathode for the flow of nonconsecutive load current impulses, and means for rendering said cathodes emissive comprising excitation anodes associated with said cathodes and a source of alternating current, the combination of a transformer comprising a primary winding connected with said source and a secondary winding, a plurality of inductive circuits connecting said secondary winding with said excitation anodes and said cathodes, and means for simultaneously rendering said cathodes nonemissive to thereby interrupt the flow of said load current impulses comprising means responsive to an operating condition of said valves for simultaneously opening said inductive circuits.

8. In a control system for a plurality of electric valves each comprising a single main anode and acathode for the flow of nonconsecutive load current impulses, means for rendering said cathodes emissive comprising excitation anodes associated with said cathodes and a source of alternating current, the combination of a transformer comprising aprimary winding connected with said source and a secondary winding, connections between said secondary winding and said excitation anodes, a connection between said secondary winding and said cathodes, and means for simultaneously rendering said cathodes nonemis sive to thereby interrupt the flow of said load current impulses comprising means responsive to an operating condition of said valves for simultaneously opening the first said connections.

9. The combination of a plurality of interconnected electric valves, each of said valves comprising a single main anode, a cathode, excitation anode means and a control electrode, means for rendering and maintaining the different said cathodes continuously emissive comprising a source of current connected with said cathodes and with the different said excitation anode means to supply the different said cathodes with continuous excitation currents, and means for rendering said valves nonconductive comprising means for simultaneously interrupting the different said excitation currents to render the difierent 11' said cathodes non-emissive and for substantially simultaneously therewith impressing a deionizing potential on the different said control electrodes to prevent accidental reestablishment of the flow or current through any of said valves.

10. The combination of a plurality oi. interconnected electric valves, each of said valves comprising a single main anode, a cathode, excitation anode means and a control electrode, means for rendering and maintaining the difierent said cathodes continuously emissive comprising a source oi! current connected with said cathodes and with the diflferent said excitation anode means to supply the different said cathodes with continuous excitation currents, means for impressing variable control potentials on the different' said control electrodes to control the conductivity of the diflerent said valves and means for rendering said valves non-conductive comprising means ifor simultaneously interrupting the diflere'nt said excitation currents to render the different said cathodes non-emissive and for substantially simultaneously therewith substitutin a deionizing potential for said variable control potentials to prevent accidental reestablishment of the flow of current through any of said valves.

SAMUEL R. DURAND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

